1. Field of the Invention
The present invention relates to a valve apparatus for opening and closing a fluid flow passage. In particular, the present invention relates to a valve apparatus which is characterized by a coupling structure between a valve stem and a drive unit.
2. Description of the Related Art
An exhaust gas-recirculating valve has been hitherto used, for example, for removing harmful components discharged from the internal combustion engine. The exhaust gas-recirculating valve has a function to make communication between the intake system and the exhaust system of the internal combustion engine. The harmful components such as NOx contained in the exhaust gas are reduced by recirculating the exhaust gas discharged from the internal combustion engine to the intake system.
In general, the exhaust gas-recirculating valve includes a valve comprising a valve plug which is used to open and close a recirculating passage for making communication between the intake system and the exhaust system of the internal combustion engine. The valve plug is displaced by operating a drive unit by the aid of a valve stem to perform the opening and closing operation for the valve.
A coupling structure between a drive unit and a valve stem is disclosed, for example, in FIG. 1 of U.S. Pat. No. 5,020,505 (hereinafter referred to as "first illustrative conventional technique"). In the first illustrative conventional technique, a projection formed on an inner surface of a plunger is interposed by two disks which are secured to the valve stem so that the drive unit is coupled to the valve stem. A spring washer intervenes between the projection and the disk.
Another coupling structure between a drive unit and a valve stem is disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 8-114277 (hereinafter referred to as "second illustrative conventional technique"). In the second illustrative conventional technique, a lower end of a plunger is allowed to abut against a washer which is secured to the valve stem so that the drive unit is coupled to the valve stem. A spring intervenes between the washer and the plunger.
Still another coupling structure between a drive unit and a valve stem is disclosed in FIG. 3 of U.S. Pat. No. 5,460,146 (hereinafter referred to as "third illustrative conventional technique"). In the third illustrative conventional technique, a lower end of a plunger is allowed to abut against an upper surface of a retainer which is secured to the valve stem by means of caulking so that the drive unit is coupled to the valve stem.
Still another coupling structure between a drive unit and a valve stem is disclosed in FIG. 1 of U.S. Pat. No. 5,593,132 (hereinafter referred to as "fourth illustrative conventional technique"). In the fourth illustrative conventional technique, a projection formed on an inner surface of a plunger is interposed by the aid of a spring washer between two members which are engaged with the valve stem so that the coupling is achieved. The lower limit position of the plunger is regulated by allowing a lower end of the plunger to abut against a rib formed on a bottom surface of a sleeve.
In the first illustrative conventional technique, the plunger and the valve stem are fixed to make displacement in an integrated manner. For this reason, when the valve stem is subjected to the valve-closing operation, the inertial force of the plunger is applied to the valve stem. Therefore, it is feared that when the valve plug coupled to the valve stem abuts against a valve seat, a strong shock is exerted between the valve plug and the valve seat, which may cause a damage.
On the other hand, in the second illustrative conventional technique, a predetermined period of time is required from the start of displacement of the plunger to the abutment of the plunger against the washer. For this reason, an inconvenience arises in that a slight time lag appears in the action of opening and closing the valve.
Further, in the third illustrative conventional technique, the retainer is caulked to the valve stem in a complicated manner. For this reason, the coupling operation for the retainer and the valve stem is complicated.
Furthermore, in the fourth illustrative conventional technique, the members for interposing the plunger are engaged with the valve stem. For this reason, a complicated operation should be performed to produce and assemble the respective members. The plunger and the valve stem are coupled to one another so that they are displaced in an integrated manner. For this reason, when the valve stem is subjected to the valve-closing operation, the inertial force of the plunger is applied to the valve stem, in the same manner as in the first illustrative conventional technique described above. Therefore, it is feared that when the valve plug coupled to the valve stem abuts against a valve seat, a strong shock is exerted between the valve plug and the valve seat, which may cause a damage. The lower limit position of the plunger is regulated by the abutment between the plunger and the rib formed on the bottom surface of the sleeve. Therefore, it is feared that the lower end of the plunger is damaged upon collision with the rib. When a coating layer is formed on the surface of the plunger, it is feared that the coating layer is peeled off.